Physical Properties of Water
Chemical and physical properties of water are often discussed together.
These properties are fundamentals of many disciplines such as hydrology,
environmental studies, chemical engineering, environmental engineering,
civil engineering etc. They are of interest to chemists and physicists
Here are some highlights of the physical properties of water.
Pure liquid water has a high heat capacity of 4.182 J K-1
g-1; it is a good heat conductor, but a poor electric conductor.
It is a solvent for dissolving ionic and polar substances, but interact
with non-polar substance weakly.
Surface tension of water is rather high, and little quantities aggregate
into drops rather than spread out as thin layers.
Hydrogen bonding contributes to many of the physical and chemical properties,
such as the unusual but normally known freezing point of 273.15 K, and 373.15 K
respectively. The critical temperature and pressure are 647.3 K and 220.5
bar (22050 kPa) and critical volume = 0.056 m3 kmol-1.
Because of the many applications of water, some more details on the
properties of water are desirable. Thus, keeping track of water properties
is of national interest. For example, the
American Society of Mechanical Engineers
(ASME) is such an organization. International co-operation on research
and information exchange are more economical, and for thermal properties, The
International Association for the
Properties of Water and Steam (IAPWS) is set up. Canada is a member
of this organization.
From the application point of view, the variations of the following
properties as functions of temperature and pressure are required.
We will discuss some of these to illustrate the point, but not all of them.
- Compressibility of steam and water as a function of pressure at various
- Density of water as a function of temperature
- Viscosity of water as a function of temperature
- Enthalpy of water for various thermodynamic evaluations
- Molar volumes and expansion coefficients of water and vapor
as functions of pressure and temperature
- Speed of sound in water and vapor and speed of sound in
air-vapor mixture as functions of temperature and pressure
- Entropy of water as a function of temperature and pressure
- Thermal conductivity of water and steam
- Viscosity of water at any temperature for pipe and pump design
- Dielectric constant as functions of temperature and pressure
- Surface tension as functions of temperature and pressure
- Gibbs energy at various temperature and pressure
- Properties such as dielectric constants and ion products of
supercritical water (fluid)
- Applications of supercritical water: Plastic waste recycle,
recovery of toluenediamine, hydrolysis of PET (polyethylene terephthalate)
Density of water
Density is the mass per unit volume. The density of water
is usually taken as 1.0 g/mL or 1.000e3 kg m-3 at 277 K.
This suggests that the density varies with temperature and water density is
the highest at 277 K, and the density between 273 and 281 K from the
CRC Handbook of Chemistry and Physics are given in the Table here.
These data are calculated from experimental data for pure water based
on the standard at 276.98 K. The same source gives the density of
ordinary water as 1.000000 g/mL at 277 K.
|Density of Water
|T /K ||Density g/mL||D2O
|273 ||0.999841 ||1.10469
|277 ||0.999973 ||1.1057
|278 ||0.999965 ||1.10562
The volume occupied by one mole of substance is called the
molar volume. The molar volume of liquid water is 18.016/density.
At 277 K, the molar volume is 18.016 mL. For liquid water, the molar volumes
of liquid water increase to 18.03 mL at both 269 K and 285 K.
The density of ice is 0.917 at 273 K, and the molar volume is 19.65 mL,
9% more than the molar of liquid. Thus, 9% of an ice cube containing no air
bubble float above the surface, and 91% of it is below the waterline.
The density makes behavior of
Ice bergs are
major tourist attractions in Newfoundland and Labrador, Canada.
Electric dipole moment and dielectric constant
Charged ions interact with each other due to electrostatic attraction or
repulsion. The force F between two charge particles with charges
q1, and q2 separated by a distance
F = ------------
4 p eo
Uncharged molecules still interact with each other, not due to electrostatic
interaction, but due to electric dipole interaction.
The electric dipole moment is a vector due to uneven distribution of
unlike charges. In diatomic systems, the magnitude of the electric
dipole moment can be estimated as the difference between the
Pauling electronegativities of the two atoms. For convenience, let us
assume that centres of positive and negatives of Na-Cl are separated
by a distance l, then the electric dipole moment,
m = q l
Traditionally, the dipole moments of molecules have been tabulated
in electrostatic units, in which case the charge of an electron
is 4.80e-10 esu (= (1.6e-19 C) (3e9 esu/C)).
In NaCl crystals, the distance between Na and Cl ions is 240 pm.
If the NaCl molecule (in a gas) has the same distance between the
ideallized ions, then the dipole moment is calculated below:
m = q l
= (1.60e-19) (240e-12 m)
= 3.84e-29 C m.
or in cgs-esu units
In the cgs-esu unit, 1e-18 esu cm is define a Debye (symbol D).
Thus, we have
1 D = 1e-18 esu cm
m = 4.8e-10 esu * 2.40e-8 cm
= 11.5e-18 esu cm
= 3.355e-30 C m (from the calculation above)
m = 11.5 D for ideallized NaCl gas molecule
mobserved = 9 D in NaCl gas
=3e-29 C m.
However, experimental dipole moment = 9 D for NaCl gas. Thus, the
model for the calculation has to be modified to account for the partial
delocalization of the charges or by including some covalent character in
the Na-Cl bond. In any case, the model shows a physical method (model)
for the evaluation of dipole moment. Dipole moments of some gas molecules
are given in the table here.
|Electric Dipole Moment|
of Some Gas Molecules
The dipole moment is a vector from the negative to the positive
charge along the bond. For triatomic moecules such as those of water,
the total electric dipole moment is the sum of al dipoles for each bond.
The experimental dipole moment for water is 1.8 D, which is the same
as that of H-F. Water is a very polar compound. Ammonia with three
N-H bonds has a dipole moment of 1.5 D.
Homonuclear diatomic molecules have zero dipole moment, of course.
So are linear CO2 and CS2 molecules.
The high dipole moment makes water a very special substance.
Water has a very high dielectric constant, 80. Due to
dipole-charge interaction, water is the universal solvent for ionic
substances, especially mono-valent ions. The diagram below shows
some typical ion-dipole and dipole-dipole interaction in solutions.
The dissolution in water is called
The dipole moment has something to do with its interaction with microwave.
Application of Ground Penetrating Radar in Glaciology
is a web site that gives the theory of radar and discusses the interaction
of radar with water. It further illustrates the application in glaciology.
In contrast, supercritical water has a low dielectric constant, making it
a good solvent for non-polar substances.
Phases of water
Phase transitions are physical changes (reactions). In
Water Chemistry, we have already discussed the
ten phases of ice. Phase transition of ice under various temperature and
pressure is a physics subject.
The Kobe Steel, Ltd has a wonderful site discussing the phase transition
of liquid. In particular, the supercritical fluid of water and its
applications are discussed in detail.
- Above certain temperature, a substance cannot be liquidfied, no matter
how high a pressure is applied. This temperature is called critical
temperature. The pressure to liquify a liquid at the critical
temperature is called the critical pressure. The fluid above
critical temperature is called supercritical fluid. This fluid
has properties of liquid and gas, and its properties are very different
New Solvents gives a general introduction to supercritical water.
Reaction solvent effect (1): "The dielectric constant"
discusses the variation of dielectric constant in water including
supercritical water. Find out more about the variation.
Reaction solvent effect (2): "The ion product"
discusses the variation of the ion product,
Kw = [H+] [OH-],
with temperature and how this influences the application of water
under various conditions. See also
Using supercritical water to
recycle plastic waste is a commercial enterprise. A good example
is the recovery of toluenediamine, a reagent used in the waste recycle.
Hydrolysis of PET (polyethylene terephthalate) is another example
of the applications of supercritical water.
Let us assume the H-F molecule as composed of two ions, a positive
and a negative ion. What is the distance that separating these two ions
in order to give a dipole moment of 1.8 D?
m = q l
Recall the first formula from the discussion of dipole moment above,
and proceed with the calculation as shown:
q l = 4.8e-10 esu * l
= 1.8e-18 esu cm.
1.8e-18 esu cm
l = ----------------- = 0.375e-8 cm (37.5 pm)
What is the normal H-F bond distance? (Ans. 92 pm).
Dividing the apparent bondlength of 38 pm by the observed bondlength 92 pm
gives the ionic character 38/92 = 41 % ionic character.
Confidence Building Questions
In a fire or nuclear power generator, steam at 600ºC under pressure
is often used to transfer the heat from the boiler to the turbine.
What phase is this steam?